Major When an authenticated user attempt to access a password protected course, he is asked to enter the password or ask the password to the course teacher.
Clicking on the teacher's name, the authenticated user is prompted he cannot access the teacher's profile since he as not the needed capability. Thus, authenticated user are unable to become students since they cannot ask the course password.
The workaround is enabling the capability for Authenticated users, anyway the default logic behind the current Moodle behaviour seems quite crippled and should be fixed.